What happens when bulk insulation is installed between joists?

The Added R-Value of insulation is not the same as the Material R-value when bulk insulation is installed between the joists. The Added R-Value is is reduced by thermal bridging at the joists where the insulation level is reduced or in the case of batts missing entirely. This creates thermal bridges (or short circuits) where the Added R-Value will be considerably lower than the Material R-value. These calculations are for timber joists where thermal resistance Rj is still significant.

¹Steel joists may have significantly poorer performance especially if they are not covered. The problems with steel joists are that they are connected to steel truss and the area in contact with the ceiling is much larger than the cross secion used in these calculations. These factors combined with the high thermal conducivity mean that effective R values may be significantly lower than those produced by these calculations. The calculations also presume that the section will be filled with thermal insulation as is the case with loose fill.

Loose fill (k) W/m·K Curtin 2005
Joist (k) W/m·K ASHRAE 1993
Joist centres mm
Joist height mm
Joist width mm
Plaster board K·m²/W k = 0.16 W/m·K
Ceiling air films K·m²/W AIRAH2ed p6.17
Tile Roof w/o RFL K·m²/W
Tile Roof with RFL K·m²/W
http://www.energy-imaging.com.au/

This is what happens if joists are not covered

Loose-fill
________________________       In roof air film = 0.11 K·m²/W
~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~| Loose-fill insulation covering
[X]     [X]     [X]      ~|  | 95 mm high joists
======================== ----- Plaster + air film = 0.17 K·m²/W
|_|     | 48 mm wide joists at
|_______| 600 mm centres

Batts
________________________       In roof air film = 0.11 K·m²/W
: :~~~~~: :~~~~~: :~~~~~ ~~~~| Batt insulation over, but not covering
[X]     [X]     [X]      ~|  | 95 mm high joists
======================== ----- Plaster + air film = 0.17 K·m²/W
|_|     | 48 mm wide joists at
|_______| 600 mm centres
Loose-fill and Batt R-Values with Ru=0.28 92% Rj=0.96 8% Rs=0.30 Rt=0.00 Rf=0.26
All values are calculated from the Material R Value (R) in the left most column.
Material Ceiling System R-Value Added R-Value Tile Roof System w/o RFL Tile Roof System with RFL
R Value (R) Equiv. Batt Loose Fill (Rl) Batts (Rb) Loose Fill (Rl-Rs) Batts (Rb-Rs) Loose Fill (Rl+Rt) Batts (Rb+Rt) Loose Fill (Rl+Rf) Batts (Rb+Rf)
0.00 0.00 0.297 0.297 0.000 0.000 0.297 0.297 0.557 0.557
0.50 0.50 0.792 0.792 0.495 0.495 0.792 0.792 1.052 1.052
1.00 1.00 1.247 1.247 0.950 0.950 1.247 1.247 1.507 1.507
1.50 1.50 1.666 1.666 1.369 1.369 1.666 1.666 1.926 1.926
2.00 2.00 2.054 2.054 1.757 1.757 2.054 2.054 2.314 2.314
2.50 2.50 2.413 2.413 2.116 2.116 2.413 2.413 2.673 2.673
3.00 3.33 2.959 2.748 2.662 2.451 2.959 2.748 3.219 3.008
3.50 4.27 3.500 3.060 3.203 2.763 3.500 3.060 3.760 3.320
4.00 5.30 4.026 3.351 3.729 3.054 4.026 3.351 4.286 3.611
4.50 6.45 4.543 3.624 4.246 3.327 4.543 3.624 4.803 3.884
5.00 7.76 5.055 3.880 4.758 3.584 5.055 3.880 5.315 4.140
5.50 9.28 5.564 4.122 5.267 3.825 5.564 4.122 5.824 4.382
6.00 11.04 6.071 4.349 5.774 4.052 6.071 4.349 6.331 4.609
6.50 13.13 6.577 4.563 6.280 4.266 6.577 4.563 6.837 4.823
7.00 15.65 7.081 4.766 6.785 4.469 7.081 4.766 7.341 5.026
7.50 18.74 7.585 4.958 7.289 4.661 7.585 4.958 7.845 5.218
8.00 22.62 8.089 5.140 7.792 4.843 8.089 5.140 8.349 5.400
8.50 27.65 8.592 5.312 8.295 5.016 8.592 5.312 8.852 5.572
9.00 34.43 9.094 5.477 8.797 5.180 9.094 5.477 9.354 5.737
9.50 44.06 9.596 5.633 9.299 5.336 9.596 5.633 9.856 5.893
Loose Fill
A form of insulation that is conveyed pneumatically which fills the space. (Meets AS 3999—1992 3.2.a.i & 4.2.b)
Batt
A form of segmented or pre-formed insulation installed only betteen the joists. (Such installation does not conform to AS 3999—1992 3.2.a.i)
Cellulose insulation
A loose fill insulation derived from wood based paper.
Material
The R-value of the material itself at a given thickness.
System
The R-value of the system above where insulation of a given material R-value has been installed.
Equivalent Batt
R value to be used if estimating loose fill with estimating programs that assume batt R values.
Added
The difference in R-values between the insulated case and the uninsulated case. This value is suitable for adding to similar uninsulated cases.

See the further reduction in system R-value if we put in downlights that are not covered.

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